This invention relates to a system for the feedback control of the air/fuel mixture ratio for an internal combustion engine, which system includes an air/fuel ratio detector having an oxygen-sensitive element of the oxygen concentration cell type operated with a DC current to establish a reference oxygen partial pressure in the element, and more particularly to an improved current control means for controlling the supply of current to the oxygen-sensitive element.
In recent internal combustion engines and particularly in automotive engines, it has become popular to control the air/fuel mixture ratio precisely to a predetermined optimal value by performing feedback control with the object of improving the efficiency of the engine and reducing the emission of noxious or harmful substances contained in exhaust gases.
For example, in an automotive engine system which includes a catalytic converter positioned in the exhaust passage and containing a three-way catalyst, that can catalyze both the reduction of nitrogen oxides and oxidation of carbon monoxide and unburned hydrocarbons, it is desirable to control the air/fuel mixture ratio to a stoichiometric ratio since the catalyst exhibits its highest conversion efficiencies in an exhaust gas produced by the combustion of a stoichiometric air-fuel mixture, and also because the employment of a stoichiometric mixture ratio enhances the mechanical and thermal efficiency of the engine. It is known to feedback control the air/fuel ratio in an engine system by using a sort of oxygen sensor, installed in the exhaust passage upstream of the catalytic converter, as a device that provides an electrical feedback signal indicative of the air/fuel ratio of the air-fuel mixture actually supplied to the engine. Based on this feedback signal, a control circuit commands a fuel-supply apparatus, such as electronically controlled fuel injection valves, to control the rate of fuel feed to the engine so as to nullify or minimize deviations of the actual air/fuel ratio from the intended stoichiometric ratio.
Usually the above mentioned oxygen sensor is of an oxygen concentration cell type utilizing an oxygen ion conductive solid electrolyte, such as zirconia stabilized with calcia, and conventionally the sensor comprises a solid electrolyte layer in the shape of a tube closed at one end, a measurement electrode layer porously formed on the outer side of the solid electrolyte tube and a reference electrode layer formed on the inner side of the tube. When there is a difference in oxygen partial pressure between the reference electrode side and measurement electrode side of the solid electrolyte tube, this sensor generates an electromotive force between the two electrode layers. As an air/fuel ratio detector for the above mentioned purpose, the measurement electrode is exposed to an engine exhaust gas while the reference electrode on the inside is exposed to atmospheric air utilized as the source of a reference oxygen partial pressure. In this state, the magnitude of the electromotive force generated by this sensor exhibits a great and sharp change between a maximally high level and a very low level each time the air/fuel ratio of a mixture supplied to the engine changes across the stoichiometric ratio. Accordingly, it is possible to produce a fuel feed rate control signal based on the result of a comparison of the output of the oxygen sensor with a reference voltage which has been set at the middle of the high and low levels of the sensor output.
However, this type of oxygen sensor has disadvantages such as the significant temperature dependence of its output characteristics, the necessity of using a reference gas such as air, the difficulties in reducing the size and its insufficient mechanical strength.
To eliminate such disadvantages of conventional oxygen sensor, U.S. Pat. No. 4,207,159 discloses an advanced device comprising an oxygen-sensitive element having an oxygen concentration cell is comprising of a flat and microscopically porous layer of solid electrolyte, a measurement electrode layer porously formed on one side of the solid electrolyte layer and a reference electrode layer formed on the other side on a base plate or substrate such that the reference electrode layer is sandwiched between the substrate and the solid electrolyte layer and macroscopically shielded from the environmental atmosphere. Each of the three layers on the substrate can be formed as a thin, film-like layer. This device does not use any reference gas. Instead, a DC power supply means is connected to the oxygen-sensitive element so as to force a constant DC current (e.g. of a current intensity of about 10 .mu.A) to flow through the solid electrolyte layer between the two electrode layers to thereby cause migration of oxygen ions through the solid electrolyte layer in a selected direction and, as a consequence, establish a reference oxygen partial pressure at the interface between the solid electrolyte layer and the reference electrode layer, while the measurement electrode layer is made to contact an engine exhaust gas. Where the current is formed to flow through the solid electrolyte layer from the reference electrode layer toward the measurement electrode layer, there occur ionization of oxygen contained in the exhaust gas at the measurement electrode and migration of negatively charged oxygen ions through the solid electrolyte layer toward the reference electrode. The rate of supply of oxygen in the form of ions to the reference electrode is primarily determined by the intensity of the current. The oxygen ions arriving at the reference electrode layer are deprived of electrons and turn into oxygen molecules which result in accumulation of gaseous oxygen on the reference electrode side of the concentration cell. However, a portion of the accumulated oxygen molecules diffuse outwardly through microscopical gas passages in the solid electrolyte layer. Therefore, it is possible to maintain a constant and relatively high oxygen partial pressure which can serve as a reference oxygen partial pressure at the interface between the reference electrode layer and the solid electrolyte layer by the employment of an appropriate current intensity with due consideration of the microscopical structure and activity of the solid electrolyte layer. Between the reference and measurement electrode layers of the oxygen-sensitive element then is generated an electromotive force, the magnitude of which is related to the composition of the exhaust gas and the air/fuel ratio of the mixture from which the exhaust gas is produced. The oxygen-sensitive element may be operated by forcing a current to flow therein from the measurement electrode layer toward the reference electrode layer. In this case a constant and relatively low oxygen partial pressure can be maintained at the interface between the reference electrode layer and the solid electrolyte layer.
To supply a DC current of an accurate and constant intensity, use is made of a constant current supply circuit including conventional electronic control means.
Sensors of the type disclosed in U.S. Pat. No. 4,207,159 has advantages over prior sensors in that they require no reference gas, they can be produced a very small size and exhibit good resistance to mechanical shocks and vibrations.
In performing feedback control of the air/fuel ratio in the engine system discussed above, utilizing an air/fuel ratio detector according to U.S. Pat. No. 4,207,159 (and also in the case of using a conventional oxygen sensor), it is usual to interrupt the feed of fuel to the engine under certain operating conditions of the engine such as an abrupt deceleration condition, with a view to avoiding wasteful consumption of fuel. Under such conditions, there occur a considerable augmentation of oxygen concentration in the exhaust gas and lowering of the temperature of the exhaust gas and of the oxygen-sensitive element, with a resultant tendency of the solid electrolyte layer in the element lowering its activity. This results in an increase in the amount of oxygen ions supplied to the reference electrode layer relative to the amount of oxygen molecules diffusing outwardly through the pores in the solid electrolyte layer, even though the intensity of the DC current supplied to the element is kept unchanged. Consequently, the magnitude of an oxygen partial pressure on the reference electrode side becomes far greater than the initially intended value. When this oxygen partial pressure continues to rise beyond a certain critical level, there is a strong possibility of breakage of the oxygen-sensitive element which is comprises very thin layers. A similar tendency is apparent when the air/fuel ratio becomes exceedingly high, even though the feed of fuel is not completely interrupted and/or when the exhaust gas temperature becomes very low during operation of the oxygen-sensitive element.